The main purpose of the cross-referencer, spxref, is to find
undefined predicates and unreachable code. To this end, it begins by
looking for initializations, hooks and public directives to start
tracing the reachable code from. If an entire application is being
checked, it also traces from user:runtime_entry/1. If individual
module-files are being checked, it also traces from their export lists.
A second function of spxref is to aid in the formation of module
statements. spxref can list all of the required module/2
and use_module/2 statements by file.